Characterized by high diffraction efficiency and environment-independent stability, a holographic photopolymer material is a recording medium that stores holographic information by using the laser holography technology. Its precursor typically comprises a photoinitiating system, polymerizable monomers, agglomerant, plasticizer, and other functional components (including but not limited to, liquid crystals, SiO2, TiO2, POSS, and carbon nano-tubes). It is obtained by illuminating the precursor with two or more coherent laser beams. During the process of preparing the holographic photopolymer material, the gelation process of the polymer network competes against the diffusion process of monomers and other functional components. Perfect phase separation can be attained for preparing a holographic photopolymer material with high-diffraction efficiency only when the diffusion rate is higher than the gelation rate. To overcome the gelation problem during the polymerization,
Natarajan et al adopted the thiol-ene click reaction to control the gelation time of the photopolymerization system (Chem. Mater. 2003, 15 (12): 2477-2484; Macromolecules 2007, 40 (4): 1121-1127), but that reaction didn't afford perfect phase separation and high diffraction efficiency. After discovering that the free radicals generated by ultraviolet lights can inhibit the addition polymerization initiated by visible lights, Scott used the free radicals generated by 364 nm lasers to reduce the photopolymerization reactivity initiated by 473 nm lasers, thus effectively deferring the gelation time of the polymerization (Science 2009, 324 (5929): 913-917). However, this approach involves lasers of two different wavelengths, thus incurring additional equipment costs. To overcome this problem, a photoinitiating system is invented to improve the phase separation of the holographic photopolymer material by deferring the gelation time in the photopolymerization. The photoinitiating system allows a single laser beam to simultaneously generate two different types of free radicals, one of which initiates photopolymerization and the other one inhibits photopolymerization, making it possible to continuously adjust the gelation time in the photopolymerization and to prepare a holographic polymer material with high diffraction efficiency.